fix webgl canvas: take margin changes into account
fix webgl canvas: take margin changes into account

bloomberg/bqplot,SylvainCorlay/bqplot,dmadeka/bqplot,SylvainCorlay/bqplot,ChakriCherukuri/bqplot,ChakriCherukuri/bqplot,ChakriCherukuri/bqplot,SylvainCorlay/bqplot,bloomberg/bqplot,dmadeka/bqplot,bloomberg/bqplot